Conventional types of magneto-optical recording and reproducing units (hereafter, "MO drivers") typically use a disk made from a magnetic material such as GdFeCo or TbFeCo. The disk is magnetized with the poles perpendicular to the disk's surface to record or store data, which can be retrieved. Specifically, this type of MO driver projects a laser beam onto a location on the disk so as to apply enough energy to bring the temperature above the Curie point. Data are recorded by applying magnetization which corresponds to the polarity of magnetization of an external magnetic field in the direction perpendicular to the disk's surface. To read out the data recorded on the disk, the Kerr effect is utilized. This refers to the fact that the plane of polarization of the reflected laser beam will be rotated slightly in accord with the direction of magnetization of the disk. The varying strengths of the polarized P component and the polarized S component of the light reflected off the disk are detected.
FIG. 1 is a block diagram illustrating an example of a relevant prior art MO driver. The example shown is the type of MO driver which modulates the magnetic field. In FIG. 1, the disk 1 is rotated by a spindle motor 2. An optical system 100 projects a laser beam onto the recording side of the disk 1 (the bottom side of the diagram). A magnetic circuit system 300 generates an external magnetic field on the non-recording side of the disk (the top side in the diagram). The optical system 100 is moved along the radius of disk one by a carriage (not pictured). The optical system 100 comprises a semiconductor laser diode (hereafter "LD") 11, which emits a laser beam; a collimator lens 12, which renders the divergent light emitted by the LD 11 into a virtually parallel luminous flux; and a grating element (hereafter grating) 13, which causes the luminous flux to diverge along numerous rays by diffraction. The light which passes through the grating 13 transmits to an objective lens 15 by way of a beam splitter 14 and focused onto the disk 1.
The grating 13 is needed when the well-known three-beam method is used as the tracking servo mechanism. If a single-beam tracking servo mechanism, such as a push-pull device, is adopted, the grating 13 can be eliminated.
The light reflected off the disk 1 is transmitted to the beam splitter 14 by way of the objective lens 15. The portion of the light which is reflected off the original optical path strikes a .lambda./2 plate 16, and its plane of polarization is rotated 45.degree.. This reflected light is refracted using a condensing lens 17 and a cylindrical lens 18, in which the light undergoes a point-spread aberration when it passes through the cylindrical lens 18. The light is then transmitted to a polarizing beam splitter (hereafter "PBS") 19. The PBS 19 transmits, what is labelled for reference as, the polarized P component of the reflected light and reflects the polarized S component. The PBS 19 thus splits the light or beam into its components, which are separately focused onto light detecting elements 20 and 21, respectively.
FIG. 2(a) shows the relative position of the light detecting elements 20 and 21 and the PBS 19 as viewed from direction A in FIG. 1. To read data recorded on the disk 1, the signal differences between the output of the light detecting element 20 and the output of the light detecting element 21 are computed. One of the elements (in this example, element 20) is segmented in an appropriate manner, and the spatial distribution, intensity, and other characteristics of the received beam are used to generate the necessary error signals for focus control and tracking control servos.
The portion of the optical system 100 from the collimator lens 12 through the beam splitter 14 to the objective lens 15 is called the condensing optical system. The portion travelled by the beam from the time it is reflected off the beam splitter 14 until it reaches light detecting elements 20 and 21 is called the photodetector optical system.
The magnetic circuit system 300 consists of a coil 31, which supplies the external magnetic field, and a driver 32, which supplies the current that flows through the coil 31. The driver 32 controls the polarity of the current it supplies to the coil 31 based on the modulating signal generated by an encoder 33.
There are problems, however, with the above-described prior art MO driver. For example, as was discussed above, the optical read/write heads in the prior art MO drivers use the PBS 19 to split the reflected light into virtually orthogonal paths. The light detecting elements 20 and 21 have to be positioned in the paths of the split beam. This not only makes it problematical to set up and adjust the light detecting elements, it also makes it impractical to downsize the optical read/write head.
One possible solution may be to eliminate the .lambda./2 plate 16 for purposes of downsizing the read head. However, if this were done, the plane of polarization would tilt with respect to the PBS 19, which would have to be tilted in response thereto. If the PBS 19 were tilted, for instance, at an angle of 45.degree., it would require a vertical spacing with a height of h2 as shown in FIG. 2(b). This height h2 requires a greater space occupation and thus is not desirable in comparison to the former height h1 (the vertical spacing required when .lambda./2 plate 16 is used) shown in FIG. 2(a).
A second possible solution is to downsize the read head by placing the photodetector optical system between the LD 11 and the collimator lens 12. Specifically, the beam splitter 14 would be placed between the LD 11 and the collimator lens 12, as shown in FIG. 3. The beam splitter 14 would split the light reflected off the disk 1, and the split reflected light would be transmitted to the PBS 19.
However, if the configuration such as that shown in FIG. 3 is chosen, the light detecting elements 20 and 21 would interfere with each other, and the light detecting element 21 would also interfere with the beam splitter 14 (crosshatched areas in FIG. 3). The reason for this is the spacing requirement of the various components and panels required to enable positioning and mounting of the photodetector optical system. This difficulty would severely limit design freedom. There is also a possibility that the mounting panel on the light detecting element 21 would obstruct the light emitted by the LD 11.
To prevent the optical elements from interfering with each other, one might consider using a concave lens 22, as shown in FIG. 4. However, this too would result in a larger optical read/write head; and the increase in required components and adjustment processes would inflate the cost thereof.
Still another solution might be to use the two light detecting elements 20 and 21 arranged in a same plane. That is, a Wollaston prism 23 shown in FIG. 5 would be substituted for the PBS 19 of the embodiment shown in FIG. 1. Then the polarized P component and the polarized S component would be split along nearly the same direction. If the paths of the polarized P and S components exiting the prism 23 were lengthened, then light detecting elements 20 and 21 could be arranged side by side in the same plane. However, the Wollaston prism 23, which is required in this embodiment, is quite expensive. Furthermore, the angle .theta. formed by the flux of the polarized P component and that of the polarized S component separated by the Wollaston prism 23 would only be 1.degree..+-.0.03.degree.. Because the angle is so small, it is not possible to place the light detecting elements 20 and 21 next to each other unless the elements are sufficiently distant from the prism 23. Ultimately, this scheme would not allow the optical read/write head to be made smaller.
As can be seen in FIG. 3, the length of the optical path is limited by the focusing distance of the collimator lens 12. This would also make use of the Wollaston prism 23 extremely difficult.